1. Field of the Invention
The present invention relates to pipets for dispensing and aspirating a fluid medium. More particularly, the present invention relates to a pipet having an improved tip or dispensing portion which is constructed to reduce the tendency for dripping.
2. Description of Related Art
In laboratory and industrial settings it is well known to use a pipet to extract a certain volume of a fluid medium from one container and to transport and dispense some or all of the extracted volume into another container. Depending upon the nature of each given application, the volume of fluid medium to be extracted and dispensed must be measured accurately. Most pipets employ an open-ended tapered tip through which the fluid medium is drawn and extracted. During such operations, it is critical that the pipet not drip so that accurate volumes are dispensed. Moreover, in certain industrial or scientific manipulations where precise volume deliveries are critical, such as for example, in molecular biology applications including DNA sequencing, in situ hybridization, and enzyme digestion and in certain tissue culture applications, dripless pipet tips are essential.
Typically pipets are generally hollow tubular members which are used by applying suction at an open upper end or mouthpiece in order to extract or aspirate a quantity of fluid medium into the hollow tube. A pressure differential maintained by closing the mouthpiece opening retains the fluid within the pipet allowing transport of the fluid medium to another container. Selective opening of the mouthpiece allows a quantity of the fluid medium contained in the pipet to be dispensed. A certain degree of accuracy in the amount of fluid dispensed is provided by the tapered end portions by reducing the amount of fluid lost due to dripping.
Plastic pipets have largely replaced glass pipets for many uses. Commercially available plastic pipets, however, have a serious draw-back. Plastic pipets typically have a molded or a drawn dispensing tip in the shape of a hollow cone with an open, narrow end. Such tips tend to drip or leak, especially in larger pipets, in which the opening of the cone typically has a greater taper angle to allow for speedy dispensing of larger volumes. Dripping and leaking can be reduced by decreasing the size of the opening or narrowing the taper angle. However this would restrict liquid flow therethrough to an unacceptable level.
Given the nature of pipets and the need for accuracy in the volume of medium which is dispensed, it is common to employ different sized pipets for different volume applications. Large volume pipets, those having a capacity in excess of 50 ml are, quite naturally, of larger size than small volume pipets, those having a capacity under 50 ml.
In many large volume pipets, the size of the dispensing tip is such that it is difficult to control the dispensing of fluid from the pipet. Thus, even upon creation of the pressure differential by closing the mouthpiece, fluid medium has a tendency to drip from the mouthpiece resulting in lack of control in the accuracy of dispensing.
Accordingly, it is desirable to provide a pipet having a dripless tip for use in accurately dispensing specified volumes of a fluid medium which is easy to manufacture and to use.